They vary depending on which year and type, M.W. or street 426
I've never sonic tested the outside but I do know of some old time SO CA class racers that had the outsides machine thinner to be able to put more weight in the trunk and in the gas tanks
I'm pretty sure that they are not real thick, maybe between .160 to .220 depending again on where on the block your measuring and the how many times that sand casting mold had been used before that block was cast